20 



KNOWLEDGE. 



[January 1, 1894. 



No one can doiibt that prominences, and particularly 

 eruptive prominences, are closely related to faculip. To 

 this point I have already called attention in the following 

 words : " . . . In a great many photographs taken 

 with the spectro-heliograph, faeulii? are shown projecting 

 above the sun's limb. And the intimate relationship 

 between faculfe and eruptive prominences is not less 

 evident, especially in composite photographs showing 

 faculaj and prominences on the same plate. When we 

 consider that rrnptiir prominences probably rise from 

 faculae, it is not at all surprising that such prominences 

 sometimes show a continuous spectrum in addition to 

 their bright lines. For a violent eruption would naturally 

 carry up with the prominence some " dust-like " matter 

 from the facula, which would give a continuous S23ectrum." 

 {Astronotny and Astni-l'hi/.iii-s, November, 1892, p. 815.) 



The projection of faculse at the limb, while very 

 frequently shown in short exposure photographs" of the 

 sun's disc, is rarely much greater than the average depth 

 of the chromosphere. If faculse are prominences seen in 

 projection, or if they are always covered by prominences, 

 as Father Sidgreaves and M. Deslandres hold, these 

 projections should be much higher — i.e., we should always 

 find a prominence above one of these projecting facula^. 

 As a matter of fact, the long exposure spectro-heliograms 

 of the chromosphere rarely show prominences at such 

 points ; when prominences are present they are almost 

 invariably eruptive, and of small extent at the base. But 

 the projecting facuhe give the reversed H and K lines, 

 even when no prominence is present. M. Deslandres 

 states, however : " Les facules sont, par definition, les 

 plages brillautes de la surface solaire. plages qui, ii 

 I'intensite gent'rale pres, donnent les memes raies noires 

 que les parties voisines, et correspondent aux parties 

 elevces, aux montagnes de la photosphere. Elles sont 

 distinctes des flammes de calcium audessus d'elles." 

 (Knowledge, December, IHOB, p. 230.) 



It is probably true that the faculs are ordinarily quite 

 different from the prominences which sometimes cover 

 them, but I cannot see that any evidence, other than a 

 " definition," is oftered to prove the absence of the bright 

 H and K lines from their spectrum. These lines may 

 have their origin in hot calcium vapour distributed through 

 the mass of the facula, or confined to its outer portion, I 

 but I by no means consider it proved that the spectrum of 

 faculfB does not contain the bright H and K lines. 



Leaving for a moment the question of facula>, let us 

 next consider whether prominences projected on the solar 

 disc should be rendered visible by the spectro-heliograph. 

 The reasoning which has already led us to the conclusion 

 that the nearly continuous reversals of the H and K lines on 

 the disc originate at the base of the chromosphere would 

 seem to apply with greater force to the base of promi- 

 nences, for here the H and K lines are apparently brighter 

 than in the surrounding chromosphere. And, in fact, I 

 have previously shown that certain outbursts on the solar 

 disc, which there is every reason to believe, are true 

 eruptive prominences, have been photographed at the 

 Kenwood Observatory with the spectro-heliograph (see 

 Axtmnomy and A.stiv-Fliysic.s, 1892, p. 611 ; ihid. p. 920, 



* Obtained with the spectro-heliograph and also by direet exposure 

 at the focus of a telescope. 



t It may eren be that the lines originate in the ehromosjihere 

 overlying the faeiJie. In this ease the inereased brightness of the 

 lines, as eonipared with their brightness in other parts of the ehronui- 

 spherc, would have to he accounted for. Any such brightcnino', 

 if due to the faeuloe, woidd in all probability be confined to those parts 

 of the chromosphere immediately overlying the faculfp, so that the 

 foi'm of the latter would still be obtainecl in s]iectro-heliogr,ams. 



Plate xlvi., photographs of the eruptive prominence of -July 

 15, 1892 ; ihid, 1893, p. 454). Such brilliant outbursts 

 are quite exceptional, only seven having been found in over 

 two thousand spectro-heliograms. 



While it is thus certain that some prominences can 

 be detected on the sun's disc, it is equally certain that 

 others cannot. Since .January, 1892, bright regions in 

 which the H and K lines are reversed have been found 

 only in or near the sunspot zones. Not having access 

 to our records, I cannot give the northern and southern 

 boimdaries of this facula zone with accuracy, but I do 

 not think we have photographed a single bright calcium 

 region more than 70° north or south of the equator. 

 During the same period our photographs have shown great 

 numbers of prominences of higher latitude than 70"', and 

 prominences have not been uncommon in the near vicinity 

 of the poles. Thus there are bright prominences which give 

 no indication of their presence when projected on the disc, 

 for by no flight of the imagination could the small and 

 evenly-distributed meshes of the facular reticulation be 

 supposed to represent the bases of such large and brilliant 

 prominences. In the face of this difficulty I prefer to wait 

 for further evidence before adopting the conclusion that 

 quiescent prominences in the sunspot zones can be photo- 

 graphed when projected on the disc. 



M. Deslandres has suggested that the reversals of 

 calcium on the solar disc be called " dommi's faculuiri's, 

 nom qui est en accord avec les faits, et evite toute ambi- 

 guitc " I Knowledge, December, 1893, p. 231 ; Coinptes 

 ivndus, November 27th, 1893). I regret that, for the 

 following reasons, I cannot consistently adopt this name : — 



1. The use of the identical term " flame," commonly 

 employed to describe one variety of chemical combination, 

 is objectionable because we do not know that the solar 

 and terrestrial phenomena referred to are in any way 

 similar. 



2. I have oftered evidence to show that many faculas 

 are not covered by prominences, but themselves give the 

 H and K lines. 



3. Even if it could be shown that all facula? are covered 

 by prominences, it would seem unnecessary to replace the 

 well-known term "prominence" by a less satisfactory 

 synonym. 



For the present, if one does not wish to commit him- 

 self by speaking of faculte and prominences on the disc, 

 the general term "calcium reversals" may perhaps be 

 used, though it is not altogether free from objection. 



As to the electric origin of the bright H and K lines in 

 the sun (Deslandres lac. cit.), it seems to me that the 

 merely negative evidence at our disposal is not a safe 

 foundation on which to build an argument. It is true 

 that the hydrogen spectrum has not hitherto been 

 obtained in the laboratory by the simple efi'ect of heat, 

 but it does not follow that this gas v,-ould not give a 

 spectrum of bright lines when subjected to solar conditions. 

 The H and K lines of calcium had not been obtained 

 artificially without electrical means until I succeeded in 

 photographing their feeble radiations in certain flames 

 (see Antronnmi/ and A.stro-FIu/sini, 1893, p. 452). At 

 solar temperatures these radiations may be greatly 

 strengthened, and the gradual shift toward the violet of 

 the maximum of intensity in the calcium spectrum noticed 

 with increased temperatures renders such a strengthening 

 probable. 



While it seems quite possible, and even probable, that 

 electricity plays some part in solar phenomena, the 

 evidence upon which to base any very positive statements 

 appears to be lacking. 



Before passing on to the discussion of certain practical 



